Cl Surface

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J. Phys. Chem. C 2009, 113, 21713–21720

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The Structure and Vibrational Spectrum of the Si(111)-H/Cl Surface Glen Allen Ferguson,† Sandrine Rivillon,‡,| Yves Chabal,§ and Krishnan Raghavachari*,† Department of Chemistry, Indiana UniVersity, Bloomington, Indiana 47405, Department of Chemistry, Rutgers UniVersity, Piscataway, New Jersey 08502, and Department of Materials Science and Engineering, UniVersity of Texas at Dallas, Richardson, Texas 75080 ReceiVed: July 13, 2009; ReVised Manuscript ReceiVed: October 27, 2009

In this work, the vibrations of mixed coverage Si(111)-H/Cl surfaces are determined using quantum chemical calculations and Fourier transform infrared spectroscopy. The structure and symmetry considerations of the modes are used to assign the vibrational frequencies for varying coverages of both hydrogen and chlorine on the surface. Significant shifts in the Si-H and Si-Cl stretching frequencies are found as a function of coverage, while the bending modes are shifted very slightly. Our results suggest that Si-H stretching shifts can be used as a probe for chlorine coverage during the reaction of chlorine precursors with the hydrogen-terminated Si(111) surface. Finally, an analysis of coverage dependence suggests that a chemical inductive effect is the dominant origin of the resulting stretching frequency shifts, though a small contribution from the lone pairs interacting with the surface is also possible. I. Introduction The chemistry of functionalized silicon surfaces is an area of great scientific interest due to the ubiquitous use of silicon in microelectronics devices.1 Functionalized silicon surfaces offer the possibility of extending the range of applications of semiconductor materials with tailored physical and chemical properties.2-12 However, the determination of surface structure and properties after functionalization is a challenging task. A powerful approach to this problem is the use of surface infrared spectroscopy to characterize the surface.13,14 Although experimental spectra may be analyzed to determine surface structures in favorable cases, the technique is most powerful when spectra are interpreted using accurate quantum chemical calculations.6,15,16 Using this composite approach, previously ambiguous and difficult spectra can be definitively interpreted. In particular, the combination of theoretical calculations and experimental observations has been used successfully to understand semiconductor surface spectra for many adsorbates on the Si(100) surface. Relatively small cluster calculations treating a single dimer or a pair of adjacent dimers have been found to be adequate in most cases.15 Interestingly, such cluster models are somewhat deficient for adsorbates such as methyl groups on the Si(111) surface, particularly at high coverage where they undergo unphysical deformations if optimized fully.17 We have recently developed techniques to calculate the vibrations of these surfaces that combine periodic boundary conditions (PBC) optimization with efficient cluster vibrational calculations.17,18 Thus far, the technique has only been applied to uniform monolayers of a single adsorbate (Cl, H, D, CH3, etc.). To extend the range of applicability of such methods, we now investigate the Si(111)-H/Cl surface and compare the results to experi* To whom correspondence should be addressed. E-mail: kraghava@ indiana.edu. † Indiana University. ‡ Rutgers University. § University of Texas at Dallas. | Current address: Global Technology Center, Air Products and Chemicals Inc., Allentown, Pennsylvania 18195.

mental data to gain a complete understanding of mixed coverage surface spectra. Organic derivatives are some of the most promising functional groups currently being studied for modifying silicon surfaces. These molecules have many novel and interesting properties along with a vast body of literature, making them attractive molecules for functionalizing surfaces. The most basic functionalization is the alkylation of the Si(111) surface.19-28 Alkylation can be accomplished by a three-step procedure, starting with the bare Si(111) surface. The surface is hydrogenpassivated, that forms the nearly atomically unreconstructed 1 × 1 surface, which is much simpler than the 7 × 7 surface formed in vacuum.29,30 The surface is then chlorinated using a reagent, such as PCl531-37 or molecular chlorine,38 and finally alkylated using a Grignard-type reagent.7,21,22,26,27,33,39 Initial studies were carried out with methyl termination, whereas subsequent studies have examined unsaturated groups, such as acetylenyl and methylacetylenyl. Such functional groups can presumably be used for further functionalization to attach more complex organic molecules or biomolecules on silicon surfaces. Whatever the subsequent functionalization, the initial chlorination process is critical to the chemistry of these surfaces. During the chlorination process, the surface is partially covered by chlorine and hydrogen. The goal of chlorination is to replace the nearly ideal hydrogen monolayer with a nearly ideal chlorine monolayer. Knowledge of the percent surface coverage would allow a deeper understanding of the functionalization process and give us a tool for controlling the reaction and allowing for process optimization. An accurate determination of surface coverage is inherently difficult. Although it is possible to use intensity to measure coverage, if the frequencies shift with coverage, with accompanying changes in intensities, it is difficult to make definitive determinations. Computationally, if the relationship between the IR frequency shift and the percentage of surface coverage is established, it is not necessary to determine coverage by inexact means. Coverage could be quantitatively determined from shifts in the surface IR spectrum. In this paper, we investigate the relationship between IR vibrational frequencies and coverage using the Si(111)-Cl/H

10.1021/jp906614e  2009 American Chemical Society Published on Web 12/03/2009

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J. Phys. Chem. C, Vol. 113, No. 52, 2009

Figure 1. Si(111)-Cl surface showing the unit cell as spheres on the far left with the cell repeated as circles over the supercell. The chlorine atoms are represented by dark spheres and the silicon atoms by gray spheres.

surface as an example. The experimental observations provide evidence for coverage dependence of surface vibrations. PBC models with a range of partial coverage are used to derive the appropriate geometric and vibrational parameters. The relationship between the PBC vibrations and cluster model vibrations is discussed. The frequencies of the Si(111)-H/Cl surfaces observed in the experimental spectra are assigned. The origin and implications of coverage on the vibrational frequencies are discussed. II. Experimental Details N-type silicon(111) samples (Si) are cleaned by sequential SC1 [H2O/H2O2/NH4OH (4:1:1) by volume] and SC2 [H2O/ H2O2/HCl (4:1:1) by volume] at 80 °C for 10 min, with thorough rinsing in deionized water after each step. This cleaning protocol removes organic and metallic contaminants and leaves a clean, thin SiO2 layer on the surface. Hydrogen termination is then obtained by a brief immersion in hydrofluoric acid (HF) (10-20 wt %), followed by a 2 min ammonium fluoride (NH4F) etching.29 The chlorination of these freshly prepared Hterminated silicon surfaces is then performed at atmospheric pressure in a clean stainless steel reactor that is continuously purged with pure nitrogen (N2(g) with O2 impurity